1. Field of the Invention
This invention relates to the electrolytic recovery of metals and, more particularly, to apparatus for electrolytically recovering a metal from a solution containing ions of the metal in which an electrolytic cell is operated under negative pressure.
2. Description of the Prior Art
The technique of electrolytically recovering a metal from a solution which contains ions of such metal is well known. Regardless of the design of the apparatus employed, it has long been recognized that effective agitation of the solution must be provided to achieve a reasonable efficiency of recovery. Many prior art electrolytic cells utilized in metal recovery apparatus provide mechanical agitation of the solution by the use of inter-electrode stirring devices, e.g., rotating paddlewheels or impellers positioned between an anode and a cathode in the cell. Such cells are typically of rather complex construction and relatively large, since space must be provided between the cell anode and the cathode to accommodate the stirring devices.
Recently, smaller, sealed electrolytic cells of relatively simple design have been developed which produce a turbulence in the electrolytic solution flowing through them by directing the flow along a path of a particular geometry. The solution agitation resulting from this turbulence is sufficient to eliminate the need for using mechanical stirring devices in the cells. Examples of cells of this type are described in the following U.S. patents: A. C. Cooley, U.S. Pat. No. 3,728,244, issued Apr. 17, 1973; and J. S. Zankowski, U.S. Pat. No. 3,751,351, issued Aug. 7, 1973. Cells such as, for example, the Cooley and Zankowski cells, have in the past been operated under a positive pressure. The Zankowski cell consists of a housing that holds an anode and a cathode in contact with the opposite edges of a spiral-like or involute partition. The involute partition directs solution flow along a spiral path through the cell, between the anode and cathode, and produces sufficient agitation of the solution to allow efficient electrolytic plating. Since solution agitation is produced in a Zankowski cell by directing a solution along a spiral path defined by a partition, solution agitation and, hence, the efficiency of the cell, is reduced if part of the solution flowing through the cell leaks between partition sections, instead of following the spiral path defined by the partition. Obviously, the probability of such leakage occurring is increased when the solution is forced through the cell under positive pressure, as is done in the prior art. The pressure created by forcing the solution through the cell tends to produce a bulge in the cell enclosure, and this may result in the anode or cathode, or both, separating from the edges of the involute partition, producing an opening through which the solution leaks between sections of the partition. To eliminate this problem, the cell is constructed of relatively expensive, rigid materials, which add to the cost of producing a cell. Additionally, another problem resulting from operating a cell under positive pressure is the safety hazard that can arise if the cell housing fractures or bursts, and debris and solution are hurled outwardly from the cell as a result of such pressure. In summary, while the prior art practice of using a positive pressure to force a solution through an electrolytic cell produces satisfactory results, this practice requires the use of a relatively expensive cell and presents a potential safety hazard in the event the cell housing is damaged.